1. Field of the Invention
This invention relates to the field of logging-while-drilling (LWD) well boreholes, and more particularly relates to compensation for effects in LWD formation density measurements.
2. Background of the Related Art
The density of formations penetrated by a well borehole is used in many aspects of the petroleum industry. More specifically, formation density is combined with measurements of other formation properties to determine gas saturation, lithology, porosity, the density of hydrocarbons within the formation pore space, properties of shaly sands, and other parameters of interest.
Methods and apparatus for determining formation density, comprising an isotopic gamma ray source and two gamma ray detectors, were introduced in the 1960's by J. S. Wahl et al (J. S. Wahl. J. Tittman and C. W. Johnstone, "The Dual Spacing Formation Density Log", Journal of Petroleum Technology, December, 1964). These basic concepts are still used today, and are often referred to as dual spaced density logs or gamma-gamma density logs. The apparatus is configured as a logging tool (sonde) for conveying, preferably with a multiconductor cable, along a borehole thereby "logging" formation density as a function of depth. The source and two detectors are typically mounted in an articulating pad device with a backup arm. The backup arm applies force to the articulating pad to maximize pad contact with the wall of the borehole. The sonde responds primarily to radiation which is emitted by the source and scattered by the formation into the detectors. The scatter reaction is primarily Compton scattering, and the number of Compton scattering collisions within the formation can be related to electron density of materials within the formation. Through sonde calibration means, a measure of electron density of the formation can be related to true bulk density of the formation.
Since the dual spaced density measurement technique is based upon a nuclear process, statistical error is associated with the measurement. There is also non-statistical error in the measurement. Although the articulating pad and backup arm tend to position the pad against the borehole wall, the largest source of non-statistical error is generally still associated with the position of the tool within the well borehole, and is generally referred to as standoff error. The responses of the two detectors are combined in prior art dual spaced density systems using well known algorithms to minimize standoff error, but unfortunately these algorithms do not completely eliminate this source of error.
The dual spaced density system is now available as an LWD system. As in the wireline version of the system, the dominant non-statistical error that arises in LWD formation density measurements results from tool standoff. The standoff problem is far more complex in LWD systems than in wireline systems. The LWD tool must rotate with the drill string, therefore, the articulating pad and backup arm used in the wireline embodiment is impractical in the LWD embodiment. More specifically, standoff complexities arise from LWD tool non-concentric rotation to the borehole, linear radial tool motion relative to the borehole, and variations in the formation density surrounding the borehole in a plane perpendicular to the tool's rotation.
One approach used to resolve the LWD density measurement standoff problem is set forth in U.S. Pat. No. 5,473,158 to Jacques M. Holenka et al. Counts from the two detectors are segregated into angular (azimuthal) segments as the tool rotates in the borehole. If it is assumed that there is no radial tool motion within the borehole, tool standoff would be nearly constant over small angular segments. Count rates from the detectors, recorded in each angular segment, can then be combined using a known correction algorithm (spine and rib) to obtain a standoff corrected density measurement for each segment. The spine and rib method, described in Wahl, provides the basis for a suitable correction algorithm. Unfortunately, the radial position of the tool can vary significantly during multiple rotations through each specific segment thereby introducing standoff error if detector responses are depth shifted.
U.S. Pat. No. 5,091,644 to Daniel C. Minette discloses a error minimization technique for combining azimuthally segmented density measurements to arrive at a density value best representing the collection of segmented measurements at a given depth within the borehole. This technique assumes that formation density is constant in the plane perpendicular to the tool's rotation. This assumption is invalid when the borehole penetrates relatively thinly laminated, dipping beds or when the borehole is deviated from the vertical through thinly laminated beds. Azimuthal averaging of data measured in dipping beds will yield erroneous density values and also show bed thicknesses on a one-dimensional display which do not represent true bed thickness. Furthermore, Minette does not account for variations in the radial position of the tool when depth shifting detector responses to compute a borehole compensated density value.